1. Technical Field
The present invention relates in general to waste material treatment and more particularly to a decanting apparatus and methods for removal of a supernatant from a vessel, such as a sequential batch reactor.
2. Background Art
In a sequencing batch reactor process of wastewater treatment a reactor is operated in a batch treatment mode involving a fill phase, aerate or react phase, a settle phase and a decant phase. During the aerate phase the wastewater influent is mixed and aerated. In the settle phase the suspended solids are allowed to settle typically forming a scum surface, a layer of supernatant, and a sludge blanket. In the decant phase the relatively clear supernatant is removed without drawing solids from the scum layer or sludge blanket. When in a non-decant phase it is necessary to prevent accumulation of suspended solids in the decanter that will contaminate the supernatant that is drawn in to the weir in the decant phase.
The quality of the withdrawn supernatant is affected by the design of the decanter. If during the decant phase the decanter draws in floating solids form the scum layer or material form the sludge blanket the quality of the discharged supernatant will be unacceptable as a discharge effluent. Further, if suspended solids enter the weir of the decanter during the non-decant phases of the process the solids will be discharged with the supernatant during the decant phase adversely affecting the quality of the discharged supernatant.
The prior art decanters are generally classified as fixed decanters or floating decanters. An example of a fixed type decanter is disclosed in U.S. Pat. No. 4,596,658. The fixed decanter remains under the surface of the mixed liquor wherein suspended solids may settle in the weir. It is attempted to prevent solids form settling in the weir by maintaining the solids in a laminar flow inlet. However, it is common for solids to settle in the decanter weir and be discharged with the decanted supernatant. The fixed decanter design has numerous other drawbacks. For example, the fixed decanter design requires expensive installation and precise management of the sequencing batch reactor process to work properly.
Floating decanters attempt to prevent solids or scum from entering the weir and the stream of decanted supernatant by use of a float. An example of a floating type decanter is disclosed in U.S. Pat, No. 4,695,376. The floating decanter of the 376 patent attempts to prevent suspended solids from entering the weir during the non-decant phases by sealing the weir against the float. After the settle phase, a linear actuator is utilized to lower the weir for the underside of the float thereby enabling the weir to decant the supernatant.
One drawback of the floating decanter disclosed in the 376 patent is that during the non-decant phases the suspended solids enter and accumulate in the weir due to the inability of the device to form an absolute seal. The accumulation of solids in the decanter during the non-decant phases contaminates the supernatant discharged during the decant phase. This contamination of the supernatant results in the rejection of the discharged effluent, thus requiring flushing of the lines before resuming withdrawal of the supernatant. This floating decanter further includes guideposts or moorings, extending from the bottom of the reactor, to maintain the floating decanter in a selected location and stable in the fluid.
Another example of a floating type decanter is disclosed in U.S. Pat. No. 5,104,528. This floating decanter includes a weir disposed within the annulus of a float. A linear actuator is connected between the weir and the float. During the non-decant phases the weir is maintained in a position above the surface of the liquor. After the settle phase is completed the linear actuator is activated to lower the weir to a position below the liquid surface. The decant phase is ended by raising the weir to a position above the surface of the liquid. This prior art floating decanter includes guides posts extending from the bottom of the reactor to maintain the floating decanter in a set location and maintain stability of the decanter in the liquid.
It is a desire to provide a semi-submersible floating decanter that overcomes the drawbacks of the prior art decanters. It is a further desire to provide a semi-submersible floating decanter that is able to deliver the supernatant without contamination with solids, is simple, inexpensive, requires minimal maintenance, and is failsafe.